This invention relates to a Coriolis mass flowmeter incorporating at least one measuring tube, at least one oscillator for the excitation of the measuring tube and at least one oscillation detector for registering the oscillations of the measuring tube, with an activator serving to energize the oscillator, at least one evaluation unit serving to analyze the oscillations registered by the oscillation detector, an excitation signal path including excitation signal-path devices provided between the activator and the oscillator for transmitting an excitation signal, and a measuring signal path with measuring signal-path devices provided between the oscillation detector and each evaluation unit for transmitting a measuring signal.
The invention further relates to a method for operating a Coriolis mass flowmeter, which Coriolis mass flowmeter incorporates at least one measuring tube, at least one oscillator for the excitation of the measuring tube and at least one oscillation detector for registering the oscillations of the measuring tube, whereby the energizing of the oscillator takes place by means of an activator, the analysis of the oscillations registered by the oscillation detector is performed by an evaluation unit, an excitation signal is transmitted by the activator to the oscillator via an excitation signal path featuring excitation signal-path devices, and a measuring signal is transmitted by the oscillation detector to the evaluation unit via a measuring signal path featuring measuring signal-path devices.
The above-described Coriolis mass flowmeter design and method for operating a Coriolis mass flowmeter address the type of conventional Coriolis mass flowmeters and method for operating such mass flowmeters in which one or two oscillators serve to excite the measuring tube and the mass flow is measured on the basis of a phase difference, typically determined by means of two oscillation detectors, between the oscillations of the measuring tube at mutually separate points. To that effect, the activator feeds an excitation signal to one or two oscillators via the excitation signal path featuring signal-path devices such as digital-to-analog converters and signal conditioners such as amplifiers. The measuring tube is caused to oscillate and, typically, by means of two oscillation detectors the oscillations of the measuring tube are registered, in which process the Coriolis oscillation derived from the medium flowing through the measuring tube is superimposed over the energizing oscillation, thus permitting mass flow measurements.
For the actual mass flow measurement the measuring signals registered typically by two oscillation detectors are fed, via a measuring signal path featuring measuring signal-path devices such as analog/digital converters and signal conditioners such as amplifiers, to the evaluation unit which may be physically identical to the activator. The resulting signal progression is as follows: the excitation signal travels along the excitation signal path—of which the oscillator is a part—to the measuring tube; by virtue of its interaction with the measuring tube, i.e. by causing the measuring tube to oscillate and registering the resulting oscillation that is also modulated by the Coriolis oscillation thus generated, the excitation signal becomes the measuring signal; the measuring signal registered by the oscillation detectors then travels along the measuring signal path—of which the oscillators are a part—to an evaluation unit.
Coriolis mass flowmeters with one oscillator require at least one maximally precise signal phase measurement. Errors affecting the precision of the phase measurement are caused by the inconsistent effect of the measuring signal-path devices on the measuring signal, including, for instance, the extent to which the measuring signal-path devices affect the measuring signal as a function of the prevailing temperature. In other words, a Coriolis mass flowmeter that was calibrated at a particular temperature will produce incorrect readings of the phase difference between two oscillation detectors when its operating temperature differs from that calibrating temperature.
In the case of Coriolis mass flowmeters equipped with two oscillators by means of which the measuring tube is to be energized in different oscillating modes, the situation is even more complicated. The excitation signals as well as the measuring signals must be transmitted with maximum amplitude and phase accuracy. Consequently, errors that can negatively affect amplitude and/or phase accuracy may originate in both the excitation signal-path devices and the measuring signal-path devices.